Design,Synthesis,and Biological Activity Study Of Pesticide Molecules Derived From The Norlignan Nicotlactone A

Structure optimization based on natural products is a main approach and focus for the development of new green pesticides.γ-Butyrolactone,a five-membered oxygen-containing heterocyclic compound,is widely present in active natural products and drug molecules which have a diverse range of biological activities,thus making it a privileged structure in drug development.Nicotlactone A,a norlignan compound isolated from tobacco,exhibits high antiviral activity against tobacco mosaic virus（TMV）.In this study,nicotlactone A was taken as the research object,and its high-efficiency synthesis was first studied.Then,four series（I-IV）of 167γ-butyrolactone structural derivatives were designed and synthesized based on nicotlactone A and its synthetic intermediates as leads,using strategies such as structural simplification,scaffold hopping,ring-opening,and activity splicing.The synthesized target compounds were structurally characterized using ¹H NMR,¹³C NMR,and HRMS.Biological activity assays showed thatα-hydroxy-γ-butyrolactone compounds（II）andα-methylene-γ-butyrolactone benzyl ether derivatives（III）exhibited excellent antiviral activity.The antiviral mechanism of the highly active compounds was further studied using transmission electron microscopy,isothermal titration calorimetry,and molecular docking.In addition,salicylaldehyde derivatives（IV）containingγ-butyrolactone moiety showed significant antifungal activity.Furthermore,the action mechanism of the highly active compounds was preliminarily investigated.The main research results are as follows:（i）The proposed structure of the natural product（±）-nicotlactone A was synthesized through a five-step reaction involving Morita-Baylis-Hillman,acetylation,boronic acid esterification,In（OTf）₃-catalyzed cyclization,and Mukaiyama hydration,starting from methyl acrylate.Additionally,30 analogs of nicotlactone A（I-a and I-b series）were synthesized based on the developed apporach.Bioactivity studies revealed that the 8-demethylated product I-b1exhibited comparable activities to（±）-nicotlactone A in terms of inactivation（72.8%）,curative（40.2%）,and protective（60.3%）effects against TMV at 500μg/m L.Furthermore,structure-activity relationship analysis indicated that electron-donating groups（alkoxyl groups）on the phenyl ring,were favorable for the anti-TMV activity,while the effect of 8-methyl group on theγ-butyrolactone moiety was relatively small.Therefore,the demethylated compound I-b1（L₂）with simple structure could be a lead compound for further optimization.（ii）A total of 21α-hydroxy-γ-butyrolactone compounds（II series）were designed and synthesized through the scaffold hopping strategy using the antiviral compound I-b1（L₂）as a lead.These compounds were evaluated for their inactivation,protection,and curative effects against TMV.The results showed that compound II-b15 exhibited a better inactivation effect with an EC₅₀ of 226.2μg/m L,which was comparable to that of the lead compound I-b1（228.8μg/m L）.Compound II-b12 also exhibited a better protective effect with an EC₅₀ of 382.3μg/m L,which was comparable to that of the lead compound I-b1（384.6μg/m L）;（iii）The compound L₃ with simple structure was selected as the antiviral and antifungal lead compound.A total of 61α-methylene-γ-butyrolactone benzyl ether derivatives（III-a–c）were designed via a ring-opening strategy,and their antiviral and antifungal activities were evaluated.The bioassay studies indicated that many of the target compounds possessed good to excellent antiviral activities against tobacco mosaic virus（TMV）.Compound III-b1exhibited the best anti-TMV activity（inactivation effect,88.9%;protection effect,65.8%;curative effect,52.8%）in vivo at 500μg/m L,which was significantly higher than that of the commercial virucides ribavirin and ningnanmycin.The preliminary antiviral mechanism of compound III-b1 was investigated.Transmission electron microscopy（TEM）showed that compound III-b1 could destroy the integrity of virus particles.Then,molecular docking and isothermal titration calorimetry（ITC）analysis further demonstrated that compound III-b1exhibited a strong binding affinity to the TMV coat protein with a dissociation constant（K_d）of 3.06μM,superior to ribavirin.Thus,we deduced that compound III-b1 may interfere with the self-assembly of TMV particles by binding to the TMV coat protein（CP）.In addition,compound III-c16 showed good in vitro activity against Valsa mali and Fusarium graminearum with an inhibition rate of 100%and 89.5%at 50μg/m L;（iv）A total of 55 salicylic aldehyde derivatives containing anα-methylene-γ-butyrolactone moiety were further designed and synthesized based on the outstanding antiviral and antifungal activity of the III-c series compounds,and their antiviral and antifungal activities were evaluated.The bioassay studies indicated that some compounds exhibited certain antiviral activity and many of the target compounds possessed excellent and broad-spectrum antifungal activities in vitro against phytopathogenic fungi and oomycetes,including Valsa mali,Rhizoctonia solani,and Phytophthora capsica.Especially,compound IV-c3displayed an excellent in vitro activity against R.solani with an EC₅₀ value of 0.65μg/m L,higher than that of the positive control pyraclostrobin(EC₅₀=1.44μg/m L)and comparable to carbendazim(EC₅₀=0.33μg/m L).In addition,compound IV-c3 exhibited promising protective in vivo activity against R.solani（84.1%at 100μg/m L）,which was better than that of pyraclostrobin（78.4%at 100μg/m L）.The pot experiment displayed that compound IV-c3had 74.8%protective efficacy against R.solani at 200μg/m L,which was comparable to that of validamycin（78.2%）.The antifungal mode of action research indicated that compound IV-c3 could change the mycelium morphology,increase cell membrane permeability,reduce the respiratory metabolism,and effectively inhibit the germination and formation of sclerotia of R.solani,thereby effectively controlling the disease.In summary,in this study,a concise total synthesis route for the proposed structure of nicotlactone A has been developed,and its antiviral structure-activity relationship against TMV was investigated.Based on the lead skeleton of nicotlactone A,several compounds containing aγ-butyrolactone moiety with high activity against plant viruses and fungi were discovered,which could be lead or candidate compounds for the development of novel antiviral/antifungal agents in crop protection.